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You should have 1 O2 sensor on your 2.3, my 94 only had 1 and I can't see why you'd add another. The sensor is located on the exhaust pipe before the catalytic converter on the passenger side of the engine next to the starter.
Hint! Identify the sensor, get your wrench for the sensor ready, dive around 5-15 minutes, get your truck in the air BECAREFUL! SUPPORT THE TRUCK AND THE EXHAUST IS HOT! Get the wrech on and do your best to get it loose. Running the engine gives the metal a chance to 'relax' and makes removal easier. Install is in the reverse less the driving around.